The present invention relates to a ferromagnetic resonator utilizing ferromagnetic resonance in a ferrimagnetic thin film, and more particularly to a filter device utilizing ferromagnetic resonance and suitable for use in a microwave integrated circuit (hereinafter referred to as a MIC).
There has been proposed a filter device utilizing a ferrimagnetic thin film of yttrium iron garnet (YIG) formed on a gadolinium-gallium garnet (GGG) substrate by the liquid phase epitaxial (LPE) growth process, as disclosed in U.S. Pat. No. 4,547,754 which is assigned to the same assignee as the present invention. Filter devices of this type using YIG thin film elements attract attention for use as MIC filters because of the high Q values of their resonance characteristics in the microwave frequency band, compact structure, and suitability for mass production through the selective patterning process by LPE and lithography.
An MIC band-pass filter using a YIG thin film may be constructed generally as shown in FIG. 1, for example, wherein a dielectric substrate 1 made of alumina or the like has a first main surface coated with a ground conductor 2 and has a second main surface coated with first and second microstrip lines disposed in a parallel arrangement to form input and output transmission lines 3 and 4. As shown in the aforementioned U.S. patent, both ends of each of the strip lines 3 and 4 have heretofore been connected to the ground conductor 2 by respective connecting conductors. Each ends 3a and 4a of input and output lines 3 and 4 is connected to input and output circuits respectively. Adjacent the second main surface of the substrate 1 are first and second magnetic resonance elements, i.e., YIG thin film elements 7 and 8, which are electromagnetically coupled with the respective microstrip lines 3 and 4. These YIG thin film elements 7 and 8 are produced by forming a YIG thin film on a main surface of a GGG substrate 9 by the above-mentioned thin film forming technique and patterning the film into circular lands by a selective etching technique, photolithography, for example. Extending between the first and second YIG thin film elements 7 and 8 is a third microstrip line 10 for providing electromagnetic coupling between the elements. The coupling transmission line 10 is formed on a second main surface of the substrate 9, with both ends of transmission line 10 being connected to the ground conductor 2 by connecting conductors 11 and 12.
MIC filter devices constructed as described in the above-mentioned U.S. patent are restricted to relatively low center frequencies of several GHz at most due to two major reasons as follows. The first reason is that the YIG thin film elements need to be placed at positions where the magnetic field is maximum for the purpose of magnetic coupling with each microstrip line; however this condition is not met for relatively high center frequencies. In particular, the magnetic field is maximum at the grounding end of the microstrip line and minimum at the position .lambda.g/4 (where .lambda.g is the propagation wavelength) away from the maximum position, and therefore each YIG thin film element needs to be disposed as near to the grounding end of the microstrip line as possible for good coupling at relatively high center frequencies. The propagation wavelength .lambda.g is expressed in terms of the effective dielectric constant .epsilon..sub.eff determined from the dielectric constants of the dielectric substrate 1 and GGG substrate 9 and the shape of the microstrip lines as, EQU .lambda.g=.lambda.o/.sqroot..epsilon..sub.eff ( 1)
Accordingly, the propagation wavelength .lambda.g is reduced to 1/.sqroot..epsilon..sub.eff of the free space wavelength .lambda.o. On the other hand, each YIG thin film element needs a finite volume for substantial magnetic coupling with the associated microstrip line; e.g., for a thickness of 20-30 .mu.m, the element diameter should be around 2 mm; and at a high frequency of several GHz even if the YIG element is disposed at the grounding end of the microstrip line the distance between this position and the YIG element center is comparable with .lambda.g/4, resulting virtually in the disposition of the YIG thin film elements at locations of weaker magnetic field, and accordingly resonant high-frequency coupling efficiency between the YIG thin film elements and the microstrip lines is reduced for relatively high resonant frequencies, and the insertion loss between the filter input and the filter output at the resonance frequency (which should be low) becomes relatively high. The second reason is that the intersections of the input and output microstrip lines and the microstrip line for linking the YIG thin film elements are not located at the grounding end portions where the electric field is minimal, but instead the distance between the intersections and the respective grounding ends approaches .lambda.g/4 at which the electric field is maximal as the operating frequency goes higher, which causes the capacitive coupling to increase, so that the isolation characteristics are deteriorated significantly at higher frequencies. FIGS. 2 and 3 show the insertion loss of the patented filter device as a function of the operating frequency, and it is apparent that the input/output coupling undesirably increases at frequencies above 4.5 GHz. Namely, the device propagates the input signal irrespective of the resonance of the YIG thin film elements, and does not function as a filter.
With the intention of overcoming the above-mentioned deficiencies, the applicant of the present invention has proposed a filter device in Japanese Patent application No. 59-187079, in which the microstrip lines each have one of their ends open with YIG thin film elements 7 and 8 being disposed at positions distant from open ends by an odd multiple of .lambda.g/4. A filter of this construction can have a high center frequency above several GHz as shown in FIG. 4, but it is suitable only for a fixed band or narrow band-width variable filter because of the narrow band width of the high-frequency coupling efficiency and isolation characteristics, and a broad band variable filter cannot be realized. FIG. 4 shows as a measurement result the isolation characteristics of this filter device for different input frequencies and indicates that an effective filtering function with an isolation of 40 dB or more is accomplished in a narrow band of about three gigahertz between 11.75 and 14.75 GHz.